Purpose: Human visual motion processing is usually considered to occur via feed-forward projections through V1 into dorsal extrastriate cortex (V5/MT), though evidence also exists for an early, direct midbrain or thalamic input to V5/MT which bypasses V1. Hence we used Transcranial Magnetic Stimulation (TMS) to investigate the sequence of temporal activation of V1 and V5/MT in motion processing.

Methods: Ten participants were exposed to TMS in the region of V1 and left V5/MT during performance on a 3AFCmotion direction detection task set at a 80% correct level. Stimulus was 80 dots moving coherently (2.2cm/sec) for duration 53–84ms. Double pulses were delivered with a stimulus-TMS onset asynchrony of between 0–220ms in 32ms intervals.

Results: Mean performance showed diminished accuracy during TMS to V1 and V5. Four participants showed a strong early inhibition of performance in V1 following TMS between 0–32ms while the other 6 showed a second smaller later inhibition effect at between 95–158ms post - stimulus onset. All participants showed diminished performance accuracy with TMS to V5 between 0ms and 32ms post stimulus onset only.

Discussion: This study demonstrates that accurate perception of direction of coherent motion requires activation of V5/MT within 30ms of stimulus onset suggesting that such early motion processing bypasses striate cortex.